1. Field of the Invention
This invention relates to a liquid jet recording head, and, more particularly, it is concerned with a liquid jet recording head which functions to form and eject flying liquid droplets of recording liquid for use in a liquid jet recording system.
2. Description of the Prior Art
The ink jet recording method (or liquid jet recording method) has drawn the attention of all concerned in its capability of high speed recording, with neglible noise, and in its capability of performing recording without necessitating special treatment, such as the so-called "image fixing" on plain paper.
Of various liquid jet recording methods, those as disclosed in, for example, Japanese laid-open patent application 54-51837 and German laid-open patent application (DOLS) 2843064 are peculiar in their characteristics and are different from other liquid jet recording methods in that thermal energy is caused to act on the recording liquid to obtain motive power for ejecting liquid droplets.
That is to say, the recording methods disclosed in the publications above referred to have their characteristics in that the recording liquid which has undergone action of the thermal energy brings about a change of state accompanying an abrupt increase in its volume, and this change of state creates an acting force to eject the liquid from the orifice at the distal end of the recording head, thereby forming flying droplets to be adhered onto a recording member for image recording.
In particular, the liquid jet recording method disclosed in DOLS 2843064 possesses its characteristics such that not only it is effectively applicable to the so-called "drop-on-demand" recording method, but also a full line type high density, multi-orifice recording head can be readily realized in the recording apparatus, hence an image of high resolution and high quality can be obtained at a high recording speed.
The recording head unit of the recording apparatus for use in the abovementioned liquid jet recording method is constructed with a liquid discharge section having an orifice for ejecting the recording liquid and a liquid flow path which is communicatively connected with the orifice, and has as its part a heat acting zone where thermal energy acts on the liquid for droplet discharge; and electro-thermal transducer as a thermal energy generating means.
This electro-thermal transducer is provided with a pair of electrodes and a resistive heat generating layer which is connected with the electrodes and has a region to generate heat between these electrodes (heat generating portion). The pair of electrodes is generally composed of a selective electrode and a common electrode, across which electric conduction is effected to generate thermal energy in the abovementioned heat generating portion for ejecting liquid droplets from the orifice.
In the ordinary case, a protective coating (or layer) is provided on the heat generating portion and at least on the electrode disposed underneath the region in the recording head where the recording liquid flows or stays. The protective coating is provided for protecting the electrodes and the resistive heat generating layer forming the heat generating portion both chemically and physically from the liquid thereabove, for preventing short-circuiting between the abovementioned pair of electrodes and leakage of electric current from the same type of electrodes, particularly, the current leakage across the selective electrodes, and for preventing electric corrosion of the electrodes which can take place by contact of the liquid and the electrode and by electric conduction thereacross.
The abovementioned protective coating is required to have various characteristics depending on the place where it is provided. For example, when it is provided on the heat generating portion, the protective coating is required to have (1) heat-resistant properties, (2) liquid-resistant properties, (3) liquid penetration preventive properties, (4) heat-conductivity, (5) oxidation preventive properties, (6) insulating properties, and (7) anticracking properties; and, when it is provided on other region than the heat generating portion, the protective coating is required to be excellent in its liquid penetration preventive, liquid-resistant, insulating, and anticracking properties, although these properties may be relaxed to some extent depending on the thermal conditions.
However, at the present, there is no material available for forming the protective coating which can satisfy all the abovementioned seven requirements with a single layer and yet cover the entire region on the heat generating portion and the electrodes. In the actual recording head, therefore, various materials having mutually complementary properties for the required characteristics are selected depending on the location where the protective coating is to be provided, and these materials are laminated in a plurality of layers for the protective coating. Such multi-layered protective coating is further required to have sufficiently high adhesive strength among the laminated layers, and not to bring about troubles due to decrease in the adhesive strength such as exfoliation and floating between the adjacent layers in the course of production of the recording head or during a period of its actual use.
Apart from the above, in the case of the multi-orifice type liquid jet recording head, since a multitude of very fine electro-thermal transducers are simultaneously formed on the substrate in the course of manufacturing the recording apparatus, there are repeatedly performed formation of each and every layer on the substrate or base member, and removal of a part of the layers thus formed, and, at the stage of forming the protective coating, the surface of the laminated layers on which the protective coating is to be formed has very fine surface irregularities with wedge portions (stepped portion), so that the step-coverage properties of the protective coating at this stepped portion is of importance. That is to say, if the step-coverage properties of the protective coating at this stepped portion is poor, there occurs penetration of the liquid at this portion to induce electric corrosion or dielectric breakdown. Further, when the protective coating has a sufficiently significant probability of containing defective portions therein owing to its manufacturing method, there inevitably takes place penetration of the liquid through such defective portions with the consequence that the service life of the electro-thermal transducer becomes considerably curtailed.
For the abovementioned reasons, the protective coating is further, required to have good step-coverage properties at the stepped portions, have very low probability of containing defective portions such as pin holes, etc. in the layers to be formed, or, if contained, to such an extent that they are practically negligible.
In particular, the heat acting surface undergoes very severe conditions such that vigorous temperature changing cycles are repeated between high and low temperatures in a frequency of several thousands times per second, and, at the same time, the liquid on the heat acting zone is subjected to repetitive pressure changes such that is is vaporized at the high temperature level to cause bubbling in the liquid, thereby increasing pressure in the liquid flow path, and, the vaporized liquid is condensed and the foams are extinguished with temperature decrease to lower the pressure in the liquid flow path, so that mechanical stress is constantly imparted to the heat acting zone by such repetitive pressure changes. On account of this, the protective coating to be provided for covering the top surface of at least the heat generating portion is required to be particularly excellent in its impact resistant property to the mechanical stress and adhesive property among the plurality of layers constituting the protective coating.
However, the conventional liquid jet recording heads have not been able to satisfy the abovementioned various conditions and requirements. In particular, exfoliation of the layers in the multi-layered protective coating provided on the top surface of the heat generating portion could not be prevented during use of the conventional apparatus over a long period of time, and a peeling-off phenomenon took place very often. Furthermore, the adhesive strength between the adjacent layers of the multi-layered protective coating decreases and exfoliation tended to occur easily between such adjacent layers during every process step of manufacturing the recording head such as, for example, in the step of forming the liquid flow path on the substrate with the electro-thermal transducer protected by the protective coating being provided thereon, or, in the step of severing the recording head for separating the recording head or forming the orifice, or others. It has also taken place often that balance in thickness of each and every layer for the protective coating thus formed is lost due to preference having been given on designing the protective coating so as to fully satisfy the abovementioned requirements for the characteristics of the protective coating, or very delicate variations in the conditions for laminating the layers to construct the protective coating, or other factors.